1. Field of the Invention
The present invention relates to a method of and an apparatus for displaying a color video picture by sequentially displaying color images on a display device and switching illumination light colors depending on the displayed color images.
2. Description of the Related Art
Various different processes are employed to display color video pictures on display devices such as liquid crystal display devices. For displaying a color video picture on a direct-view-type liquid crystal display apparatus, red (R), green (G), and blue (B) microcolor filters are associated with pixels arranged in a matrix to pass either one of R, G, B lights separated from white light emitted by a back light source with respect to each of the pixels, and the transmittance of the liquid crystal is modulated with a video signal at each of the R, G, B pixels. The full color display of each picture element, which is composed of three R, G, B pixels, is based on a spatial additive mixture (juxtaposed additive mixture) of color primaries of the three R, G, B pixels. According to this process, since a full color video picture can be displayed on a single liquid crystal display device, it is possible to construct a low-cost, small-size display apparatus. However, the liquid crystal display apparatus fails to make full use of the resolution of the liquid crystal display device used because the number of picture elements of a video picture displayed by the liquid crystal display apparatus is one-third of the number of pixels of the liquid crystal display device.
For displaying a color video picture on a projection-type liquid crystal display apparatus, white light emitted by a light source is divided into three color primaries of R, G, B, which are modulated by respective liquid crystal display devices disposed in the paths of the color primaries, based on a video signal, and the three color primary images are projected by a projection lens onto a screen to produce a color video picture thereon. The full color display of each picture element is based on an additive mixture (simultaneous additive mixture) at one spot of color primaries of the three R, G, B pixels. Since the number of picture elements of a video picture displayed on a screen is the same as the number of pixels of each of the liquid crystal display devices, the liquid crystal display apparatus is capable of producing a high-resolution color video picture. However, the liquid crystal display apparatus is large and expensive to manufacture because the liquid crystal display devices for generating the three color primary images are required and an optical system is needed for color separation and color combination.
Another process different from the above two processes is based on an additive mixture (successive additive mixture) of three color primaries which is achieved when the colors of R, G, B are cyclically changed at a rate beyond the limit of the time resolution of the human eye. This process is referred to as a field sequential process or a color sequential process, and displays a color video picture by cyclically changing R, G, B images at a high speed.
FIG. 1 of the accompanying drawings shows at an enlarged scale a temporal succession of displayed images, illustrative of a conventional field sequential display process. According to the conventional field sequential display process, one frame period is divided into three color fields including an R field, a G field, and a B field. To allow the observer to view displayed images without flickering, the frame frequency should preferably be 60 Hz or higher, and hence the field frequency should be 180 Hz or higher. When R, G, B images are cyclically changed at a high speed and displayed at the above field frequency, each pixel 204 in displayed image 206 looks white to the observer due to the mixing of the three colors R, G, B. A display apparatus that operates according to the field sequential display process can produce high-resolution color video pictures because the number of picture elements thereof is the same as the number of pixels of each of displayed images 206. Since the display apparatus has only one display device, the display apparatus is not large and can be manufactured inexpensively.
FIG. 2 of the accompanying drawings shows a conventional field sequential display apparatus as disclosed in Japanese laid-open patent publication No. 7-318939, for example. The disclosed field sequential display apparatus has color wheel 200 comprising sectorial R, G, B color filters and motor 202 which rotates color wheel 200 to produce R, G, B temporal illuminating lights from white light emitted by light source 207. Based on a video signal applied to the field sequential display apparatus, rotation controller 208 for controlling the timing of the colors of the illuminating lights and liquid crystal driver 203 for displaying images of the colors are synchronized to display a temporal succession of R, G, B color images on display device 201 for thereby displaying a full color video picture thereon.
Other color display apparatus include a color display apparatus for successively switching on three R, G, B light sources at a high speed to display R, G, B images in synchronism with the light sources and a color display apparatus which has a color filter unit, i.e., a color shutter, capable of cyclically changing colors in front of a black-and-white display device. These color display apparatus are advantageous in that they can produce high-resolution color video pictures as the number of picture elements thereof is the same as the number of pixels of the display device, and they have only one display device.
Another projection-type field sequential color display apparatus has a display device comprising pixels each in the form of a minute movable mirror that can be displaced to turn on and off, i.e., modulate, illuminating light.
However, the above field sequential color display apparatus suffer a problem in that since achromatic images such as white (W) images and images of intermediate colors are represented by a temporal combination of R, G, B images, if the observer abruptly moves its line of sight while a white dot or object, for example, is being displayed, then the R, G, B images formed on the retina are displaced, and their colors R, G, B fail to be mixed into white, but are perceived as being separated from each other.
FIGS. 3a and 3b of the accompanying drawings show displayed images which are illustrative of the problem of the field sequential display process. As shown in FIG. 3a, the observer can see displayed white (w) image 206 as a normal white image as long as the observer observes the image with a fixed line of sight. However, if the observer abruptly moves the line of sight horizontally, for example, then, as shown in FIG. 3b, the observer instantaneously perceives strips of primary colors R, B and intermediate colors Ye (Yellow), Cy (Cyan) on both sides of the white image.
A quick motion of the line of sight is referred to as “saccade”, and may cover up to 15° in 1/100 second when the eyeball moves fast. For example, while R, G, B images are being cyclically changed at the frequency of 180 Hz on the display screen that is 1 m spaced from the observer, if the observer abruptly moves the line of sight, then the R, G, B images are displaced 15 cm each. The same phenomenon occurs when the observer blinks rather than moving the line of sight. This phenomenon gives an instantaneous strong stimulus in the form of flickering primary color lights to the observer, making the observer feel uncomfortable and fatigued. Furthermore, when the observer swiftly moves a hand in front of displayed image 206, the shadow of the hand appears in divided colors. When the observer moves the line of sight smoothly to follow a displayed white object, since the R, G, B images thereof are slightly displaced with time differences, the edge of the object appears with colors.
The above phenomenon is referred to as “color breakup”, which is a most serious drawback to be eliminated in the field sequential color display processes and apparatus. One attempt to reduce the color breakup is to increase the field frequency of R, G, B fields. For example, the field frequency may be increased 10 times to reduce the color breakup to 1/10. However, since the field frequency becomes 1800 Hz, the frequency of a drive signal for the display device also increases, and the video signal processing requires a high-speed memory. The burden on the display device is too large for the reduction of the color breakup that is achieved.
According to another effort for reducing the color breakup, a white (W) field is added among the R, G, B fields (see Japanese laid-open patent publication No. 8-101672, for example). This scheme is effective to prevent a color breakup from occurring with respect to a white image (achromatic image) because it is displayed in white itself rather than the combination of R, G, B colors. However, images other than achromatic images still suffer a color breakup upon an abrupt motion of the line of sight because those images are displayed on the basis of R, G, B images.